1. Field of the Invention
The present invention relates to an air bag activation device which is capable of detecting faults that will cause the malfunctioning of the air bag system equipped on a vehicle, and to a method for such fault detection.
2. Description of the Prior Art
FIG. 1 shows the circuit arrangement of the conventional air bag activation device disclosed in Japanese Patent Publication No. 61-57219 for example. In the figure, reference numeral 1 denotes a battery of an automobile on which the air bag system is equipped, 2 is an ignition switch, 3 and 4 are acceleration sensors which sense the collision of the vehicle, 3a and 4a are contacts that are closed in response to the detection of the collision of the vehicle, 3b and 4b are resistors through which a small current is supplied from the battery 1 to an air bag activating means 6 in the state when the contacts 3a and 4a of the acceleration sensors 3 and 4 are open, and 5 is a reverse current blocking diode. The air bag activating means 6, which is a detonator, for example, for igniting the explosive used to inflate the air bag (not shown), is supplied from the battery 1 with a triggering current that is large enough to trigger the expansion of the air bag when the contacts 3a and 4a become closed.
Indicated by 7 is a backup capacitor which is normally charged by the battery 1 and discharges power to the air bag activating means 6 if the battery 1 fails to work, 8 is a diode through which the backup capacitor 7 discharges, and 9 is a resistor through which the backup capacitor 7 is charged.
Indicated by 10 is a differential amplifier which amplifies a voltage Vs appearing between the terminals of the air bag activating means 6, 10a is an operational amplifier, 10b and 10c are gain setting resistors having the same resistance and 10d and 10e are gain setting resistors having the same resistance, all associated with the operational amplifier 10a, 11 is a window comparator which detects a fault of the air bag activating means 6 based on the output voltage Vd of the operational amplifier 10a, 11a, 11b and 11c are resistors for setting reference voltages Va and Vb for the window comparator 11, 11d and 11e are operational amplifiers each producing a fault detection signal if the difference of the output voltage Vd of the operational amplifier 10a from the reference voltage Va or Vb exceeds a prescribed value, and 11f is a logical AND gate which turns on an alarm lamp 12 when at least one of the operational amplifiers 11d and 11e produces the fault detection signal.
Next, the operation of the foregoing circuit arrangement will be explained. Initially, when the ignition switch 2 is turned on, a closed circuit is formed by the reverse current blocking diode 5, the resistor 3b in the acceleration sensor 3, the air bag activating means 6 and the resistor 4b in the acceleration sensor 4, and a small current having its value limited by the resistors 3b and 4b is fed to the air bag activating means 6. This state is called "standby state".
During the standby state, if the contacts 3a and 4a are closed in response to the detection of the collision of the vehicle by the acceleration sensors 3 and 4, a triggering current which is larger than the small current of the standby state is supplied from the battery 1 to the air bag activating means 6 and the air bag is inflated. This state is called "expansion state". The value of the triggering current is determined from the voltage of the battery 1 and the internal resistance R of the air bag activating means 6.
The voltage Vs appearing across the air bag activating means 6 when the ignition switch 2 is on (the voltage level is different between the standby state and expansion state) is applied to the differential amplifier 10. The differential amplifier 10 amplifies the voltage Vs based on its gain parameter (the reason for this amplification will be explained later) to produce an output voltage Vd.
The window comparator 11 compares the voltage Vd with the reference voltages Va and Vb so as to detect a fault of the air bag activating means 6. Specifically, if the air bag activating means 6 has a short-circuit, causing the differential amplifier 10a to produce a virtually zero voltage Vd, the operational amplifiers 11d and 11e compare the voltage Vd with the reference voltages Va and Vb to detect a difference in excess of the prescribed value, and a logical low-level signal (fault detection signal) indicative of the occurrence of short-circuit is produced. In response to a fault detection signal produced by at least one of the operational amplifiers 11d and 11e, the output of the AND gate 11f goes low (ground) to turn on the alarm lamp 12.
Finally, the reason for the amplification of the voltage Vs with the differential amplifier 10 will be explained briefly. The air bag activating means 6 has an internal resistance which is as low as several ohms and the voltage Vs across it is too small for accurate fault detection, and therefore the voltage Vs needs to be amplified. However, the operational amplifier 10a has a property of having an offset voltage Vofs at the input (it produces a small voltage difference even for zero-volt inputs), i.e., it amplifies the sum of Vs and Vofs to produce the output voltage Vd, and therefore a fault of the air bag activating means 6 cannot be detected accurately.
In order to alleviate the influence of the offset voltage, it is desirable to feed as large a standby current as possible to the air bag activating means 6, provided that it does not trigger the expansion of the air bag. In this case, however, if the battery 1 fails to supply power for some reason, the backup capacitor 7 is required to release a proportionally large current, resulting in an increased size of the capacitor 7 and thus the whole air bag activation device.
The conventional air bag activation device arranged as described above must supply a small current to the air bag activating means 6 for its fault detection during the standby state, resulting unfavorably in an increased power consumption of the device. Moreover, fault detection of the air bag activating means 6 is based on the amplified voltage Vd which represents the small voltage Vs, and a variation in the battery output voltage for some reason causes the voltage Vd to vary, resulting in a degraded accuracy of fault detection. In case the standby current increases significantly due to the fluctuation of the battery output voltage, it might even trigger the expansion of the air bag erroneously.
The differential amplifier 10a which amplifies the voltage Vs cannot avoid air offset voltage, and a fault of the air bag activating means 6 cannot be detected accurately. The occurrence of a short-circuit to the power source or a ground-fault of the air bag activating means 6 may not produce a significant change in the voltage Vs, and in this case these faults cannot be detected.